The answer to this question can be found at the very end of chapter one. Ponyboy and Sodapop are going to sleep for the night, and they talk to each other as they are falling asleep. A lot of Soda's comments are about how Darry really does love Ponyboy, but Ponyboy doesn't really buy it. Ponyboy then changes the subject and asks Soda why he dropped out of school. Soda doesn't give a big convoluted answer. He simply says he dropped out because he thinks he dumb.
"Cause I'm dumb. The only things I was passing anyway were auto mechanics and gym."
Soda then changes the subject by telling Ponyboy his future plans about him and Sandy. He says that he loves her, and he is planning on marrying her. He then admits that he will have to wait until she gets out of high school and he gets a better job. Sodapop also thinks that he might wait long enough for Pony to get out of school too. That way Soda can help Darry with the bills until Pony is old enough to do that himself after high school.
"I think I'm gonna marry Sandy. After she gets out of school and I get a better job and everything. I might wait till you get out of school, though. So I can still help Darry with the bills and stuff."